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ABSTRACT
Acrania are bisexual animals. Their gonads are serial ductless organs bulging into the atria along both sides of the body. Both testes and ovaries are composed of non-germinal and germinal cells and are enclosed in complex narrow cavities and envelopes comprising blood vessels and myoepithelia. The male non-germinal cells are single phagocytic and secretory epithelial cells which do not form a blood-testis barrier. Spermatogonia, different stages of primary spermatocytes and spermatids, are connected by intercellular bridges. Spermatozoa have a bell-shaped acrosomal complex with, at their posterior pole, one large mitochondrium and a simple 9+2 flagellum. The ovarian non-germinal cells are interconnected by cell junctions, produce secretory granules and presumably also steroids, and have phagocytotic properties. During growth and maturation the oocytes undergo striking changes of nuclear, nucleolar, and cytoplasmic organization. Amount and distribution of nuage material, yolk granules, cortical granules, and precursors of the extracellular vitelline coat mark specific stages of differentiation in the cytoplasm.